1. Technical Field
The present invention relates to a piercing through type capacitor made of two piercing through holes, and more particularly to a piercing through type capacitor which is used in either high voltage high frequency device such as microwave oven, or large power electromagnetic wave device such as transmitter for broadcasting station or X-ray generator.
Generally, a piercing through type capacitor element used in high voltage high frequency device employs ceramic capacitor.
In ceramic capacitors, piled type ceramic capacitor has become widely used rapidly from the requirement of miniaturizing as well as becoming to larger capacity.
Ultra high frequency wave (300 MHz-3000 MHz) required at said broadcasting instrument or microwave oven and the like is normally obtained from magnetron device.
FIG. 1 is a circuit diagram of noise filter connected to power supply line of magnetron heater within normal microwave oven.
Wherein capacitors C1, C2 and inductor L1, L2 make a noise filter, and reference numeral 100 is conductive filter housing, numeral 102 represents magnetron, numeral 104 represents heater of magnetron 102, and numeral 106 is anode of magnetron 102 and said anode 106 is grounded.
Since the magnetron has high operating voltage between anode electrode and heater, said capacitors Cl, C2 should be the capacitor for withstanding against high voltage. Since said capacitors receive respectively high temperature heat within a microwave oven, they should have excellent temperature characteristics.
The present invention relates to an improved structure of capacitor used as noise filter within such microwave oven.
The capacitor of such object should be piercing through type capacitor, and two separate capacitors or twin capacitors within single housing are utilized for above-described object.
Such magnetron device obtains very high frequency (basic wave: 2.45 GHz) by applying accelerating voltage (-4.2 KV) to magnetron within shielded shielding case.
Since said magnetron should be completely shielded from exterior as far as possible, accelerating voltage should be supplied to the magnetron necessarily through the piercing through type capacitor.
2. Prior Art
Said piercing through type capacitor is typical conventional piercing through type capacitor as described in U.S. Pat. No. 4,370,698 as shown in FIGS. 2 and 3, which has elliptic ceramic 1 having a pair of piercing through holes over its thickness. A pair of separate electrodes 4, 5 respectively having a corresponding piercing through hole are provided on top surface of ceramic material 1, and common electrode having corresponding piercing through holes is provided on its bottom surface. Grounding plate 7 is provided with rectangular plate 7c and protuberance 7a. The rectangular plate 7c has four perforated holes for fixing capacitor assembly to filter housing (not shown) by utilizing screw or rivet.
The protuberance 7a has a pair of piercing through holes 9, 10 corresponding respectively to two holes 2, 3 of the ceramic material 1.
And, the protuberance 7a has a number of small piercing through holes 7b along the periphery thereof. Since the common electrode 6 is provided at the protuberance 7a of the grounding plate 7, the piercing through holes 9, 10 are superposed respectively with two holes 2, 3, and small piercing through holes 7b are located at exterior of the ceramic material 1. A pair of long rod 11, 12 have tabs 20, 21, and these are to be connected to receptacle terminals of external circuit (not shown). The rods 11, 12 are inserted respectively to the piercing through holes 9, 10 of the grounding plate 7 and two holes 2, 3 of the ceramic material 1. In order to ensure the insulation between the common electrode 6 and the rods 11, 12, the conductive rods 11, 12 are covered by insulation tubes 15, 16 made from, for example, silicone rubber and the like.
Said insulation tubes 15, 16 absorb contraction stress of injected filler 17 produced at heating and vucanizing process, and hence it serves a function for preventing the generation of cracks and the like in the volcanized filler 17.
Said rods 11, 12 are connected also electrically to metal caps 13, 14 respectively provided to the electrodes 4, 5.
And, the rods 11, 12 are respectively welded to the caps 13, 14. Further, the caps 13, 14 have respectively protuberance having a number of small holes 13a, 14a. Elliptic cylinder type hollow plastic cover 8 is attached to the bottom of the grounding plate 7 and this surrounds the rods 11, 12 and the tubes 15, 16. Lower end of the cover 8 is so designed that passes the lower end of the tubes 15, 16 and then protruding a little. The cover 8 should necessarily be elliptic hollow cylinder having a pair of parallel long linear walls 8a, 8b and a pair of semicircular walls 8d, 8e for connecting these long linear walls. The cover 8 has a bridge 8c traversing said parallel long linear walls at upper middle portion of this, and this bridge divides the cover 8 into two regions of substantially circular form. As shown in said FIG. 3, insulation filler 17 which is, for example, epoxy resin surrounds the ceramic material 1, rods 11, 12 and caps 13, 14. In the injecting process of insulation filler, the capacitor body is covered by the cover 18, and then the filler 17 is injected through the opening of the bottom of cover 8 into the cover 18. The insulation filler injected from the bottom passes through the small holes 7b provided to the grounding plate 7 and the small holes 13a, 14a provided to the caps 13, 14, and accordingly interior space region of the cover 18 is filled by the insulation filler. After said injected filler is valcanized, when the cover 18 is removed away, twin capacitor assembly is made. Consequently, not only the insulating endurance but also protection against oil, moisture and dust are ensured by said injection epoxy resin.
FIGS. 4 and 5 show some other example for the conventional piercing through hole type twin capacitor (for example, refer to Japanese Utility model laid open publication gazette No. 106330/85). Wherein elliptic ceramic material 31 has a pair of holes 32, 33 across its thickness. A pair of separate electrodes 34, 35 having the corresponding piercing through holes are provided on the top surface of the ceramic material 31, and common electrode 36 having the corresponding piercing through holes is provided at its bottom surface. Grounding plate 37 has protuberance 37a as well as rectangular plate 37c. The rectangular plate 37c has four perforated holes for fixing the capacitor to filler housing (not shown).
The protuberance 37a has substantially elliptic piercing through hole 37b. A pair of long piercing through strip rods 41, 42 formed integrally with tabs 41a, 42b at their top ends are respectively inserted through metal caps 39, 40, holes 32, 33 to elliptic hole 37b. The strip rods 41, 42 are respectively soldered to the metal caps 39, 40 provided to the electrodes 34, 35 respectively. The strip rods 41, 42 are also respectively covered by insulation tubes 44, 45 for same object as aforementioned tubes 15, 16. Elliptic cylinder type major plastic cover 38 is attached on the grounding plate 37 and it surrounds the lower portions of the strip rods 41, 42 covered with tubes 44, 45. In this structure, the bottom end of the cover should be so designed that passes the bottom end of the tubes 44, 45 and protruding a little as shown in said FIG. 5. Another elliptic cylinder type plastic cover 43 is provided on the grounding plate, and as shown in FIG. 5, it surrounds the ceramic material 31, caps 39, 40 and upper portion of the strip rods 41, 42.
The epoxy resin is injected not only to the opening of the cover 38 but also to the upper cover 43 of its interior. This is because the metal caps 39, 40 and the protuberance 37a have no small piercing holes as shown in said FIGS. 2 and 3, and accordingly the assembly is completely divided into two housings. The insulation filler 46 surrounds the exterior of the ceramic material 31 and the caps 39, 40 as well as upper portion of the strip rods 41, 42. Other insulation filler 47 surrounds the interior of the caps 39, 40 and middle portions of the strip rods 41, 42 covered with tubes 44, 45.
However, the conventional capacitor of said FIGS. 2 and 3 has following disadvantages. As described in above, the lower free end of the cover 8 is so designed that passes the bottom end of the tubes 15, 16 and protruding a little. Under this structure, even if relatively lower voltage is applied to the capacitor, electric discharge between the tabs 20, 21 and graounding plate 7 are started. Such electric discharge hurts human body, and therefore, when the capacitor is used in oily, dusty and dampish environment such as kitchen, that is, in a environment which is anticipated that large quantity of oil, dust and/or moisture may be deposited on the exposed bottom surface of the tabs 20, 21, the filler 17 and/or the cover 8, then it is serious.
And next, after the rods 11, 12 having the tabs 20, 21 are inserted to the holes 9, 10, other holes 2, 3 and metal caps 13, 14 respectively, the filler 17 is injected from the opening of the cover 8 between them. Therefore, in the filling process of the filler, there may be possibility that the insulation resin will be accidentally deposited on the surface of the tabs 20, 21. This will disturb the setting of electrical connection between the tab and receptacle terminal (not shown) to be attached to the tab.
Said conventional two piercing through type twin capacitor of FIGS. 4 and 5 has following disadvantages in addition to the advantages as the capacitor of FIGS. 2 and 3. When the capacitor is used in oily, disty and moist environment, the oil, dust and moisture will be deposited on the exposed top surface of the filler 46, exposed surface of the rods 41, 42, and interior and exterior surfaces of upper cover 43. Particularly, since some recesses is formed on the surface of the filler 46 and on the vertical wall of the cover 43, a large quantity of oil, dust and moisture are liable to be deposited on the surface of the filler. This will cause a substantial decrease of surface resistance of path P1 which extends from the rods 41, 42 through the surface of the filler 46 and the surface within the cover 43 to the grounding plate 37. That is, as shown by symbol P2 of FIG. 3, insulation creeping distance of path becomes substantially shorter by the oil, dust and moisture deposited on the recesses. Electric discharge starting voltage depends upon the insulation creeping distance. Therefore, when a large quantity of oil and the like are deposited on the recesses, the creeping distance or surface resistance is decreased, and accordingly, even when relatively lower voltage is applied to a capacitor, discharge following along said path is liable to be happened.
For these disadvantages, it can be readily thought to remove the upper cover 43 portion protruded to upward by passing through the exposed surface of the filler 46, that is, to make that the recess to be deposited with oil and the like is not produced. However, said method is not suitable because the insulation creeping distance will be decreased by the creeping distance provided by removed protrusion of upper cover 43.
And, according to the piercing through type capacitor of the conventional technique, since the upper cover 43 and lower cover 38 having the insulation function are separately structured respectively, parts of complicated structural form are increased whereby manufacturing process is difficult and complicated, and according to this, working efficiency and productivity are reduced and consequently it has been caused a factor of cost rising.
And, in case of assembling by inserting the upper cover 43 and lower cover 38 to the grounding plate 37, when dimensional error is large, damage of upper or lower cover 43, 38 is apt to be occurred, and since many of assembling parts of the piercing through type capacitor are not completely sealed upon injecting the insulation resin material, said insulation resin material may be leaked, and therefore badness of the product has been liable to be produced.